The present invention relates to an improvement in control valves, and more particularly to a control valve for use in mixing several kinds of fluids together in a predetermined ratio for controlling the flow rate of the fluid.
Control valves of the diaphragm type, which are usually used for such use, have the following drawbacks. First, the valve is small in rangeablility because a diaphragm is used (Rangeability is the ratio of the maximum flow to the minimum flow of both of which are controllable in the control valve). Second, the valve requires a compressed air source and piping for supplying the compressed air. Third, because compressed air is used, the valve is influenced by external conditions such as temperature, while the use of a spring for returning the diaphragm involves great hysteresis. Fourth, the necessity for compressed air, the diaphragm and the spring results in a low resolution (Resolution, as defined by the International Electrotechnical Commission, is the least interval between two adjacent discrete details which can be distinguished one from the other). Fifth, the valve is not amenable to remote control because the use of compressed air entails the problem of a pressure drop. Finally, the valve operation is not highly responsive to instructions because compressed air is further compressed when fed to the diaphragm.